1. Field of the Invention
This invention relates to a device for facilitating the connection of power tools having a standard three-prong parallel blade terminal, grounded power plug to an electrical power source terminating in a three-prong twist lock receptacle. More particularly, the invention relates to adapters for use between twist-lock power receptacles and parallel blade power plugs.
2. Description of Background Art
On building construction sites, electrical power required to operate drills, saws and similar construction equipment is frequently available only at the terminals of a twist-lock receptacle. The twist-lock receptacle may be fixed to a portable generator panel or be attached to the end of a power cable.
In a standard twist-lock connector assembly, the receptacle, or female portion of the assembly, is provided with three arcuate, circularly arranged slots in the flat front face of the receptacle. The male portion of the connector assembly comprises a plug which has an equal number of arcuate blades extending perpendicularly outward from the front face of the plug. The dimensions and arrangement of the blades on the plug are complementary to the slots in the receptacle, in the sense that each blade can be inserted into a corresponding slot by simple axial movement of the plug body towards the receptacle body. Then the plug body is rotated with respect to the receptacle body. Now since one or more of the blades has a tab protruding back from the tip of the blade at an angle to the plane of the blade, and the corresponding slot has a laterally extending recess adapted to receive the tab, rotation of plug and receptacle body with respect to one another causes the tab to engage the recess. In this position, receptacle and plug are locked together, and may be disengaged only by rotating plug and receptacle relative to one another in an opposite, disengaging tab and recess and permitting plug and receptacle to be pulled apart from one another.
Twist-lock plugs and receptacles of the type described are used at construction sites to prevent accidental disconnection of plug and receptacle solely by applying a tug on either receptacle or plug cord. In fact, safety regulations frequently mandate the use of twist-lock connectors in hazardous environments, to guard against the production of sparks, or the possible contact of live receptacle terminals with water which might be on the surface over which the power cord is strung.
While many construction sites have electrical power available only at twist-lock receptacles for the reasons described above, most standard power tools are supplied with an integral power cord having a standard three-prong grounded plug. In plan view, the three-prong grounded plug has two parallel, rectangular cross section blades providing source and return terminal functions, and a circular cross-section earth ground terminal located on the mid plane between the two blades, but displaced perpendicularly from a perpendicular line joining the inner parallel faces of the blades.
The standard grounded three-prong power plug used with most power tools cannot be plugged into a twist-lock receptacle. Therefore, if a construction worker wishes to use a standard power tool at a construction site, he must presently cut off the plug from the tool power cord and reconnect the power cord to a twist-lock power plug. This is generally a time consuming and therefore costly process. Furthermore, removing the standard three-prong, grounded plug and replacing it with a twist-lock plug makes the power tool unusable at other locations which do not have twist-lock receptacles available.